Much research has been devoted to coupling radiation between an optical integrated circuit and an optical fiber, in particular the case in which the size of the mode of the radiation in the optical integrated circuit and in the optical fiber differs largely. Typically, such a difference in mode size results in significant problems. For example, in the case of coupling between a typical single-mode optical fiber used in telecommunications application, which typically has a mode size of about 10 micrometers in diameter, and a semiconductor integrated waveguide used in silicon-on-insulator (SOI) chips, which typically has a mode size of about 0.4×0.2 micrometers, bridging this difference requires specific solutions, such as, for example, a mode-size converter.
One example of a mode-size converter is an SOI inverted taper with an overlying waveguide, which may be used for coupling between a special optical fiber and a SOI chip. However, such a mode-size converter does not allow for coupling to a standard fiber with low loss, as there is still a large mismatch between the mode of the overlying waveguide and a standard optical fiber. Rather, a special optical fiber must be used, such as a lensed fiber or a small core fiber having a high numerical aperture, which has a smaller mode size than standard single-mode fibers. These special optical fibers (in particular the latter) may require precise alignment for use.
Another solution for coupling radiation between an optical integrated circuit and an optical fiber is to use a taper based on a waveguide made of the buried oxide underlying the integrated optical waveguide. The latter results in a minimal theoretical loss of at least 3 dB. For generating an underlying waveguide, the silicon substrate must be etched away locally. Other types of mode-size transforming devices for coupling between an integrated waveguide, e.g. an integrated waveguide on an SOI chip, and a standard fiber are three-dimensional silicon tapers, but these require relatively complex manufacturing steps such as, for example, the deposition of thick amorphous silicon and/or grayscale etching.
Another solution for coupling radiation between an optical integrated circuit and an optical fiber is a grating coupler. Grating couplers can achieve relatively low coupling losses but typically are restricted to efficient operation in a limited bandwidth. For some applications, a bandwidth of more than 100 nm is required, which is not feasible using grating couplers.
Another solution for coupling radiation between an optical integrated circuit and an optical fiber is a lens or a lensed fiber, but that is a very expensive solution because of the required alignment tolerances.
Thus, while several mode-size converter solutions exist for SOI waveguides, the fabrication of these solutions is typically very challenging and requires special processing steps.
During fabrication, usually a plurality of optical integrated circuits are processed on the same wafer, resulting in one wafer comprising multiple, e.g. hundreds or thousands, of identical circuits. Such a wafer then is cut into individual dice using a dicing saw, resulting in a rough facet for coupling radiation. Reducing the roughness may be performed using facet polishing, but this is a delicate, tedious, and expensive process.